The present invention relates to a hydraulic control device applied to a hydraulic loading apparatus of an industrial vehicle such as a forklift. More particularly, the present invention relates to a hydraulic control device that maintains the flow rate of hydraulic fluid flowing from a high pressure circuit to a low pressure circuit of a hydraulic apparatus regardless of load fluctuations of the low pressure circuit.
Japanese Laid-Open Patent Publication No. 11-315803 disclose such a hydraulic control device. The hydraulic control device of the publication is applied to a forklift that has a hydraulic actuator. The hydraulic actuator is operated with a switching valve. The hydraulic control device is capable of sending hydraulic oil the flow rate of which corresponds to the opening degree of the switching valve to the hydraulic actuator. That is, the hydraulic actuator is connected to a high pressure circuit through the switching valve. A pump sends hydraulic oil from a tank to the high pressure circuit. When the switching valve is manipulated, hydraulic oil is supplied to the hydraulic actuator from the high pressure circuit through the switching valve, which actuates the hydraulic actuator. The switching valve and the hydraulic actuator form a downstream circuit.
The hydraulic control device of the publication has a bypass type flow control valve. The flow control valve includes a spool, a pilot chamber corresponding to one end of the spool, and a spring chamber corresponding to the other end of the spool. A spring for urging the spool toward the pilot chamber is provide in the spring chamber. A return circuit is provided to return hydraulic oil to the tank. The high pressure circuit is connected to the return passage with an oil passage. The spool is moved to adjust the opening degree of the oil passage connecting the high pressure circuit with the return circuit.
When the hydraulic actuator is being actuated, the pressure of hydraulic oil in a section upstream of the switching valve acts on the pilot chamber and presses the spool toward the spring chamber. Hydraulic oil in a section downstream of the switching valve, or hydraulic oil receiving the load pressure of the hydraulic actuator, enters the spring chamber and urges the spool toward the pilot chamber. The spool is moved to an axial position at which a force based on the pressure of hydraulic oil in the pilot chamber is in equilibrium with a force based on the pressure of hydraulic oil in the spring chamber and the force of the spring. The spool thus adjusts the opening degree of the oil passage between the high pressure circuit and the tank circuit. In other words, the flow control valve adjusts the flow rate of hydraulic oil flowing from the high pressure circuit to the return circuit in accordance with the load pressure of the hydraulic actuator, thereby compensating for the flow rate of hydraulic oil supplied from the high pressure circuit to the downstream circuit. That is, the flow control valve prevents the flow rate of hydraulic oil supplied from the high pressure circuit to the downstream circuit from being influenced by the load pressure in the downstream circuit. As a result, regardless of the load pressure of the hydraulic actuator, hydraulic oil is supplied to the hydraulic actuator at a flow rate corresponding to the opening degree of the switching valve.
In the hydraulic control device of the above publication, a flow rate compensation mechanism for the downstream circuit is formed only by the spool of the flow control valve. Therefore, the range of the flow rate of hydraulic oil supplied to the downstream circuit, in which range the influence of the load pressure of the downstream circuit is precluded, is narrow. That is, when the flow rate of hydraulic oil supplied to the downstream circuit is in a compensation range, the influence of the load pressure is precluded. However, if the flow rate is out of the compensation range, the flow rate of hydraulic oil is influenced by the load pressure. In the hydraulic control device of the above publication, the flow rate of hydraulic oil is compensated for in a small range to eliminate the influence of the load pressure.